An oriented, heat-set polyethylene terephthalate film coated with a plasticized nitrocellulose resin containing a sulfonamide-aldehyde resin



, ing material.

United States Patent AN ORIENTED, HEAT-SET POLYETHYLENE TER- EPHTHALATE FILM COATED WITH A PLASTI- CIZED NITROCELLULOSE RESIN CONTAINING A SULFONANIIDE-ALDEHYDE RESIN David Ivan Sapper, Cheek towaga, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware N0 Drawing. Application January 17, 1956 Serial No. 559,747

6 Claims. (Cl. 117--7) This invention relates to the manufacture of poly-v ethylene terephthalate film. More particularly it relates to the preparation of polyethylene terephthalate film suitable for conversion to bags, containers and similar packages.

The desired properties for packaging films are well known. They include strength, heat-scalability, durability of the heat-seal as well as durability of the complete film, moisture-proofness, low permeability to oxygen and carbon dioxide, high gloss, slip, transparency, clarity' and freedom from electrostatic charges.

Polyethylene terephthalate film, prepared in accordance with the procedure described in U. S. Patent 2,465,314 of Whinfield and Dickson, requires a special treatment to obtain strength. This treatment involves orienting the film by stretching and/or rolling followed by heating the film at constant length to set it in the oriented condition. One such process which involves stretching the film 2.5 to 3.25 times its original dimensions and setting it by heating While holding it under tension at a temperature of 150 to 200 C. is described in a copending application U. S. Serial No. 287,354, filed May 12, 1952 to A. C. Scarlett. orienting, however, produces changes in the film tending to impair some of the remaining properties of .the film. For instance, after orienting polyethylene terephthalate film it is no longer possible to heat-seal the film with most. conventional heat-sealing apparatus and obtain durable bonds. Attempting to heat-seal results in severe shrinkage with accompanying puc'kering and weakness at the seal. This is attributable to the high temperatures, 235 to 240 C., required for heat-sealing the polyethylene terephthalate film.

The industry has recognized this major obstacle to the use of polyethylene terephthalate film as a packag- Several remedies have been proposed but these remedies tend to improve heat-scalability at the expense of some other property. A polyethylene terephthalate film possessing all the optimum requirements for use in a wide variety of commercial packaging applications has remained an elusive goal.

The object of the present invention is to provide a containing nitrocellulose, a plasticizer for nitrocellulose,

and a resin of sulfonamide-aldehyde. The composition of the lacquer based on the weight of the solids therein is preferably at least nitrocellulose, at least 25% Coating weight grams/m. 5.4 Thickness inches .00068 Heat-seal strength grams 360 Water vapor permeability do 80 Tenacity lbs./in. 16,100 Elongation percent 61.8 Modulus lbs./in. 537,900 Tear strength grams 9.1 70 Impact strength kilogramscentimeter 30.8

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of a plasticizer for the nitrocellulose, and greater than 10% of the sulfonamide-aldehyde resin.

As the nitrocellulose component, it is preferred to use a nitrocellulose that can be well plasticized by plasticizers at slightly elevated temperatures and that can tolerate relatively large amounts of plasticizer without becoming tacky especially as dried. The nitrocellulose component, it has been discovered, can be any of the soluble grades of nitrocellulose containingbetween 10.5% and 12.2% nitrogen and having viscosities of at least second, preferably 3 to 6 seconds, at 25 C. as determined by formula C of ASTM Method D-301-33.

For the plasticizer, phthalates such as dibutyl phthalate and dicyclohexyl phthalate are particularly advan As the resin, it is preferred to use aryl sulfonamide formaldehyde resins known commercially as Santolite MHP 1 and Sontolite MS In a specific embodiment of this invention a base film of polyethylene terephthalate, stretched three. times its original dimensions in two directions and heat-set at 200 C. while under tension, was coated with a lacquer by passing the base film through a dip tank at a rate of about 30 meters per minute. The film prior to coating was inches wide and 0.0005 inch thick. The lacquer was prepared by using a solvent mixture containing 57.3% methyl ethyl ketone, 38.2% toluene and 4.5% ethanol. The following components were added by stirring and heating to to C. during a period of about 1 to 2 hours.

Percent Nitrocellulose (11.45%-11.65% nitrogen) Q. 49.14 Aryl sulfonamide-formaldehyde resin 13.58 Dicyclohexyl phthalate 19.03 Dibutyl phthalate M 9.97 Sulfuric acid-treated wood rosin 1 5.02 Parafiin wax 3.02 Bentonite clay 0.24

1 Polypale WG, manufactured by Hercules Powder Co.

2 BC Volclay, manufactured by C. A. Wagner Co.

It should be noted that the time and temperature used in preparing the coating lacquer must be controlled to prevent any substantial condensation reaction between the plasticizers and the resin.

The coating lacquer in the dip tank was at a temperature of 35 to 41 C. After being applied to the film from the dip tank, the coating was smoothed on the film by passing the coated film through doctor rolls set with a 4.6 mil opening. The film was then dried at a temperature between and C.

The coated film had excellent slip; was clear and transparent; was substantially moisture-proof; and possessed the physical properties given below:

lvisrnufactured byMonsanto Chemical Company.

and then placed in a dry (less thanv3% relative humidity) I air-swept ov'en'at'39-.-5 C. for 24 hours. The-assembly was removed :from the oven, cooled to room temperature andreweighed. Theweightloss wasconverted to grams of.water lost/.100 square meters/hour.

Heat-seal. strength was measured by, cutting a. piece ofathe coatedi film 4" x with .the-grainru' 'ining in thelong ;direction.into two pieces 4- x 57'. The.- two pieces were superimposed so that oppositesurfaces were incontact. The two pieces'of superimposed film were then sealedrtogether at each end-atright angles to the grain. A %1" wide sealing bar-heated to atemperature of 160 C. at 20 pounds per square inch pressure contacted the ends for two seconds. Thesealed sheets were then cut in half .at right angles to. the grain. center of the four resulting pieces, 1 /2 wide strips parallel to the grain were cut. The four. sets of strips were then conditioned for 2 to 3rdays at 75 F; and 35%" relative'humidityh They were tested by opening each. set of strips at the free ends, placing'them in'a Suter testing machine and pulling them apart. The heatsealstrengthwasihe highestforce in grams required to pull the .strips .apart.

The remaining. measurements and tests were conventional.

The invention Will be more fully-understood byreferring to the examples that follow. In the examples, parts are by weight unless otherwise specified.

EXAMPLE I This example'illustrates the critical importance of selecting the quantity of resin for use in the coatingformulation.

Coating lacquerswere prepared by using 53 partsof nitrocellulose"(11;45%11.65% nitrogen), 40 parts of 'a PIaSfiClZCI'ICOIDPOSECl IOf equal parts of dibutyl 'phthalate and dicyclohexyl phthalate, and. from 5. to 60 parts of .an aryl sulfonamide-formaldehyde resin. These. components were added to asolvent mixture of 60% methyl ethyl ketone and 40% tolueneat a temperature of 50 to 60 C. and stirred for a period oflto 2 hours. Resulting coating compositions contained'from about 8% to about 20% solids;

Polyethylene terephthalate film, oriented and heat-set in accordance with the'procedure described previously, was passed through a dip tankcontaining the coating lacquer at a temperature of about 30 C. Doctor knives set with a 3-4 mil opening smoothed the lacquer on the polyethylene terephthalate film. The coated film was subsequently dried at 100 to 120 C.

The properties of'the resultingfilms, except heat-seal strength, were substantiallyindependent of'the percentage of the aryl sulfonamide-formaldehycle resin used. The heat-seal strengths varied and the importance of using more-than 10% of the resinis brought out in Table 1. It was also discovered that no significant improvement in heat-seal strength was achieved by using more than about 25% of the resin.

2 Santolite MHB, manufactured byMonsanto Chemical Ch.

From the Tiz'bl I Resin Concentration Coating Heat-Seal Test Weight Strength (M r Parts Percent EXAMPLE J II This example illustrates thecritical specification of at least 25% of plasticizerfor nitrocellulose in the coating formaulation.

Coating lacquers were prepared in the manner described in Example I using 53" parts of nitrocellulose (11.45 %-11.65 nitrogen), 15 parts of aryl sulfonamideformaldehyde resin; 5;5 parts ofa sulfuric acid-treated wood? rosin, 3i33 p'artsi of a paraffinwax, 0.26 partot afbentonite-clay,andxfrom' 24 to 66'parts of'a plasti cizer. composed of? dibutyl .phathalateand -dicyclohex'yl phthalate. The lacquers were coated on oriented,iheatset polyethylene terephthalatefilmi by following the procedure described in Example :I.

A.comparison of the heat-seal strengths are given in Table: 2. It is concluded from these results thatat least 25%of the. plasticizer is' necessary and. thatbest results are obtained with27%*to '3'1%='of.a plasticizer. Poor slip properties in the final. film were o'bservedwhen the plastieizer' composition was increased. above 40 The other physical properties did not vary significantly. from test to test.

Table 2 Plastici'zer v Coating Heat.- Test. Ratio of j Concentration IIhick- Seal Dibutyl ness Strengt Phthalate'f (g/m2) '(grams) Dicyclohexyl (Parts) (Percent) Phthalate 1:1 24 24 9.5 30. 1:1 28 27 12.8 335' 1:1 32 29 11.0 255' 211 32 29 11.6 230 1:1 35 31' 14.0 230 1: 1 38 33" 11.5 1: 1 60 v 39 13.6 180 1:1 60 44 14:2 215 1:2 60 v 44' 15. 0 2:1 60 44 I 9.8 175 EXAMPLE III Coatinglacquers were prepared andiapplied to 4% wide;v0-.0005 inch'thick, oriented-,heat-set polyethylene: terephthalate film in themannerdescribed in Example 11.

The coating contained 48.6% nitrocellulose (11.45% 11.65% nitrogen), 14.66% dibutylfphthalate, 14.66% dicyclohexyl phthalate; 13.75 of the aryl sultonamideformaldehyde resin, 5.04% of asulfuric acid-treated wood rosin, 3.05% offa parafiin wax and 0.24% of'a bentoniteclay. The lacquer wasapplied to provide coating weightsof 11, 12, and 17 grams per square meter'of base-film. The result-s shownin- Table' 3 indicatethat the ultimate heat-seal strength is satisfactory throughout this range'o'f coatin'g -weights.'-

EXAMPLE IV I Example 111 was re-run with one changethe plasticizer contained 9.77% dibutyl phthalate and L 19.55% dicyclohexyl phthalate. Coating weightsfiof 10 and 18 grams per square meter were. applied withlthe following satisfactory results.

Table 4 Coating Heat-Seal Test Weight Strength (g./rn.=) (grams) EXAMPLE V Percent Nitrocellulose 11.45 %-1l.65% nitrogen) 49.14 Aryl sulfonamide-formaldehyde resin 13.5 8 Dicyclohexyl phthalate 19.03 Dibutyl phthalate 9.97 Sulfuric acid-treated wood rosin 5.02 Paraffin wax 3.02 Bentonite clay 0.24

After being applied to the film from the dip tank at a temperature of 35 to 45 C., the coating was smoothed on the film by passing the coated film through doctor rolls set with a 4.6 mil opening. The film was then dried at a temperature between 105 to 120 C.

The coated film displayed the following properties:

Coating weight grams/ml- 7.0 Coating thickness inches .00073 Heat-seal strength grams 490 Water vapor permeability do 95 Tenacity lbs./in. 15,750 Elongation percent 68.4 Modulus lbs./in. 548,000 Tear strength grams 7.5 Impact strength kilograms-centimeter 28.9

From the above examples it will be apparent that the coating composition for improving the heat-sealability of oriented polyethylene terephthalate film should contain at least 25% of a plasticizer and over 10% of a sulfonamide-aldehyde resin. Nitrocellulose, the main constituent of the film-forming coating should constitute from 25% to 65% of the composition.

The composition is prepared by dissolving desired amounts of the components in well known organic solvents. The particular solvent mixture must be selected to retain the solvent ingredients in a homogeneous solution until the entire solvent mixture, which must be volatile, is evaporated. Ordinarily the solvent required is sufficient to provide a solution containing from about 8% to about 20% solids. However, the solids may range from to 50%. In forming the coating lacquer, one precaution is worth mentioning again. No substantial reaction between the nitrocellulose plasticizer and the resin should be permitted to take place. Otherwise, the effect of these ingredients in the coating mixture will be substantially diminished.

Other ingredients well known to the art, such as waxes, clays, etc. may be added to the coating formulation for certain specific effects. These are usually added to im prove transparency; to provide increased surface slip;

and to control moisture-proofness. The addition of these ingredients will of course depend on the end. use contemplated for the film.

The coating lacquer may be applied in any suitable manner. In the examples the coating lacquer was applied by passing the film through a tank containing the coating lacquer. However, the coating may be applied by spraying, brushing or, if desired, it may be pressed on the base film in sheet form. The coating may be applied to one side of the film or to both sides. When coating from solutions by passing the film through a dip tank, it is usually easier to coat both sides.

The important result achieved by this invention is to impart heat-sealability to a polyethylene terephthalate film without sacrificing any of the desirable qualities such as transparency, clarity, strength, moisture-proofness, etc. of the base film. Heat-scalability, as it has been used in this specification, encompasses the ability of the film to be heat-sealed at moderate temperature C. to C.) and the resultant strength of the seal. The success of the present invention in not affecting adversely the important properties of the base film is due both to the novel coating formulation and the relatively low heat-sealing temperatures that the coating permits.

The coated films of this invention are useful principally as packaging materials for foods, cigarettes, textiles, hardware, etc. However, polyethylene terephthalate film coated with the novel formulation are useful wherever it is necessary to join such film together.

Since various changes and modifications may be made in the coated film and the process for making it without departing from the spirit and scope of the present invention, the invention is not restricted except as defined in the appended claims.

What is claimed is:

1. A packaging film comprising a base film of oriented, heat-set polyethylene terephthalate having at least one surface coated with a composition comprising at least 25% nitrocellulose having a nitrogen content between 10.5% and 12.2%, at least 25% of plasticizer for the nitrocellulose, and greater than 10% of a sulfonamide-aldehyde resin.

2. A packaging film as in claim 1 wherein the resin is an aryl sulfonamide-forrnaldehyde resin.

3. A packaging film as in claim 1 wherein at least one of the plasticizer components is selected from the group consisting of dibutyl phthalate, dicyclohexyl phthalate, benzyl butyl phthalate, cyclohexyl butyl phthalate, butyl phthalylbutyl glycolate, dibutyl sebacate, butyl benzoyl benzoate, dicyclohexyl adipate, triphenyl phosphate, and tricresyl phosphate.

4. A packaging film as in claim 1 wherein the plasticizer is a mixture of dibutyl phthalate and dicyclohexyl phthalate.

5. A process for preparing a packaging film which oomprises orienting a base film of polyethylene terephthalate; heat-setting the polyethylene terephthalate film by holding it under tension at a temperature of 150 to 200 C.; coating the heat-set, oriented polyethylene terephthalate film with a composition comprising at least 25 nitrocellulose having a nitrogen content between 10.5% and 12.2%, at least 25 of plasticizer for the nitrocellulose, and more than 10% of a sulfonamide-aldehyde resin; and drying the coated film.

"6. A process for preparing packaging film which comprises stretching a base film of polyethylene terephthalate 2.5 to 3.25 times its original dimension in at least one direction; heat-setting said polyethylene terephthalate film by holding it under tension at a temperature of 150 to 200 (3.; coating said heat-set polyethylene terephthalate film with a composition comprising at least 25 nitro cellulose having a nitrogen content between 10.5% and 12.2%, at least 25% of plasticizer containing at least one compound selected from the group consisting of dibutyl 

6. A PROCESS FOR PREPARING PACKAGING FILM WHICH COMPRISES STRETCHING A BASE FILM OF POLYETHYLENE TEREPHTHALATE 2.5 TO 3.25 TIMES ITS ORIGINAL DIMENSION IN AT LEAST ONE DIRECTION, HEAT-SETTING SAID POLYETHYLENE TEREPHTHALATE FILM BY HOLDING IT UNDER TENSION AT A TEMPERATURE OF 150* TO 200*C., COATING SAID HEAT-SET POLYETHYLENR TEREPHTHALATE FILM WITH A COMPOSITION COMPRISING AT LEAST 25% NITROCELLULOSE HAVING A NITROGEN CONTENT BETWEEN 10.5% AND 12.2%, AT LEAST 25% OF PLASTICIZER CONTAINING AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF DIBUTYL PHTHALATE, DICYCLOHEXYL PHTHALATE, BENZYL BUTYL PHTHALATE, CYCLOHEXYL BUTYL PHTHALATE, BUTYL PHTHALYBUTYL GLYCOLATE DIBUTYL SEBACATE, BUTYL BENZOYL BENZOATE, DICYCLOHEXYL ADIPATE, TRIPHENYL PHOSPHATE, AND TRICRESYL PHOSPHATE AND MORE THAN 10% OF AN ARYL SULFONAMIDE-FORMALDEHYDE RESIN, AND DRYING THE COATED FILM A T A TEMPERATURE OF 100* TO 120*C. 